Conventional 50 Hz television systems transmit one complete image every1/25th of a second, using a system known as interlace which transmits one half image every 1/50 th of a second, followed by the other half image 1/50th of a second later. Television cameras also operate using this interlace system, and so the original scene captured by the camera is displayed on a receiver with no motion problems. This is represented diagrammatically in FIG. 1. Time T is shown along the horizontal axis with the parallel arrows representing the sampling of the image which consists of alternate first FI1 and second FI2 fields. In this case, one half image is sampled every 1/50th of a second, that is the field period FIP. Horizontal position X is shown along the vertical axis, and the length of the arrows represents the position of the object as it moves. By drawing a line through the tips of the arrows, the trajectory TOO of the object may be observed. For this example, an object is depicted moving horizontally with a constant velocity.
Film, on the other hand, is a sequence of complete images, generated at a rate of one new image every 24 times of a second. When film is transmitted on television systems employing a 50 Hz field rate (e.g., PAL), the projection rate is 25 images a second to give simple compatibility with the television system. Inaccuracies in movement speed and audio pitch are ignored. This is shown in FIG. 2 using the same conventions as for FIG. 1 and a frame period FRP. The same trajectory TOO is traced by the object as it moves across the image.
When film is transmitted by television, at the assumed 50 Hz field rate using a telecine with a pull-down ratio of 2:1, the same frame of film is used to generate both interlace television fields. This results in the information carried by the second field being temporally displaced from the original by 20 ms. This is shown in FIG. 3. This displacement gives rise to two perceptible artifacts in the received image.
The first is a double image, and arises from the expectancy of the human visual system that objects move with a constant or smoothly changing velocity. The brain perceives two objects moving with the same velocity, separated by the distance which the original object travels in 20 ms. This is represented by the double trajectory DTOO drawn through the tips of the arrows. The second artifact is that of "judder" or stroboscopic-like effects, caused by the apparent updating of the position of each object only once every 1/25th of a second, a time interval sufficiently long for individual images to be perceived as such rather than combining together to give the appearance of smooth motion.